1. Field of the Invention
The present invention relates to a bonding wire inspection apparatus and method for inspecting wires, particularly the shape of balls at the ends of bonded wires, between the pads of a semiconductor chip and the leads of a lead frame.
2. Prior Art
FIGS. 15 through 18 show a sample 6 that is an example to be inspected. The sample 6 includes wires 5 that are bonded between the pads 2 of a semiconductor chip 1 and the leads 4 of a lead frame 3. In these Figures, reference numeral 7 indicates squeezed out paste that is used for bonding the semiconductor chip 1 to the lead frame 3. FIG. 19 shows one of the conventional inspection apparatuses.
As shown in FIG. 19, a vertical illuminating means 11 is located above the sample 6, which is placed on an inspection stand 10. The vertical illuminating means 11 is installed at the bottom of an optical means 12. A CCD camera 13, that consists of a CCD (photoelectric converter element) and a drive section therefor, is mounted on the top of the optical means 12. The CCD outputs an image signal that is an image of the sample 6 take in by the optical means 12. The image signal from the CCD camera 13 is processed by an image-processing arithmetic unit 14, and the shape of the wire 5 is recognized and the bonded position of the wire 5 is measured.
The optical means 12, that includes the vertical illuminating means 11 and the CCD camera 13, is provided on an X-Y table 15. Inside the vertical illuminating means 11, a half-mirror 16 is installed so that the half-mirror 16 is beneath the CCD camera 13. Also inside the vertical illuminating means 11 are a condensing lens 17 and a light bulb 18 that are installed on one side of the half-mirror 16.
The bonding wire inspection apparatus of this type is described in, for example, Japanese Patent Application Laid-Open (Kokai) Nos. 2-129942 and 3-76137.
In the apparatus shown in FIG. 19, when the vertical illuminating means 11 is switched on, the light from the light bulb 18 passes through the condensing lens 17, is reflected by the half-mirror 16, and then vertically directed onto the sample 6. Then, the X-Y table 15 is moved so that the CCD camera 13 is positioned above the object of inspection or the sample 6, and the image signal (that represents microscopic parts of the object of inspection, such as the ball and crescent at both ends of a bonded wire) obtained by the CCD camera 13 is subjected to noise removal, edge enhancement in the inspection area and enlargement or reduction of the image, etc. by means of the image-processing arithmetic unit 14, thus enhancing (restoring) the image of the microscopic parts of the object of inspection so that the image is easier to see. Afterward, the inspection and measurement operations are performed.
When the microscopic parts of the object of inspection are thus optically imaged, the magnification of the optical means 12 is set at a high value. For example, the magnification is set to be 2 to 3 times larger when a 2/3-inch CCD camera is used.
In the prior art described above, inspection is performed using the vertical illuminating means 11 so that the ball 5a appears dark as shown in FIG. 17. Accordingly, the area where the wire 5A is and where the edges of the pad 2 are appear dark as shown in FIG. 18 and FIG. 19(b), respectively, and the correct diameter of the ball cannot be detected. Furthermore, as shown in FIG. 19(b), the shadow 95 of the wire is unnecessarily reproduced by the camera; though, on the other hand, as shown in FIG. 19(c), the area 96 of the ball 5a which is beyond the outer circumference edge of the pad 2 is not reproduced.